1. Field of the Invention
The invention relates to wireless digital communications systems. More precisely, it relates to a transmit/receive technique in the context of communications using a plurality of user antennas, which antennas may belong to a single user or to a plurality of users, each user having a number of antennas that is greater than or equal to 1.
2. Description of the Related Art
In general, implementing communications of the multi-point and multi-antenna type raises the problem of sharing access to the spectrum resource, i.e. sharing the propagation channel. The challenge to be met by a multiple access technique is to enable different messages or data to be transmitted in parallel to distinct receivers while sharing a common physical resource, and while minimizing the resulting interference.
Various access techniques are presently in use in radio systems. Particular mention may be made of techniques that make use of orthogonality (in time, in frequency, or in space) in order to transmit information in parallel: time division multiple access (TDMA); frequency division multiple access (FDMA), such as orthogonal frequency division multiple access (OFDMA); code division multiple access (CDMA); or space division multiple access (SDMA).
By way of example, in the context of FDMA, there is a so-called “multiband IR-UWB” technique (cf. the article by S. Paquelet and L. M. Aubert entitled “An energy adaptive demodulation for high data rates with impulse radio”, Proceedings of IEEE Radio and Wireless Conference, pp. 323 to 326, September 2004). That is a multiband transmission technique making use of an impulse waveform; the message to be transmitted is constituted by on-off keying (OOK) pulses that are frequency multiplexed over a plurality of adjacent and disjoint frequency subbands. Impulse radio ultra-wideband (IR-UWB) signals are thus put into parallel on the frequency axis.
The TDMA, FDMA, and CDMA techniques, or indeed the “mixed” (time-frequency-code) multiple access techniques that rely on them, represent responses to the multiple access problem that are simple, but that rapidly reach their limits when the number of users increases. For example, with FDMA, it is possible to allocate a given frequency subband to only one user antenna, thereby severely limiting the spectrum efficiency of that technique.
SDMA provides a solution to the problem of the number of users, but that technique presents its own drawbacks. SDMA makes use of matrix operations, which are generally complex, that are applied to the multiple-in multiple-out (MIMO) channel between a transmit antenna array and a receive antenna array (the MIMO channel matrix has as its coefficient at the ith line and the jth column the gain of the propagation channel between the jth transmit antenna and the ith receive antenna). In addition, as is well known to the person skilled in the art, SDMA can achieve an improvement in quality over CDMA, FDMA, or TDMA only when the propagation channel lends itself to such an improvement, and when the propagation channel does not so lend itself, it can even present performance that is not as good as that of CDMA, FDMA, or TDMA. Two conditions that are necessary (but not sufficient) for SDMA to achieve a quality improvement over CDAM, FDMA, and TDMA are firstly that the signal to noise ratios at the receive antennas are high, and secondly that the rank of the matrix of the MIMO channel is greater than or equal to the number of multiplexed streams; consequently, if the number of streams to be multiplexed is equal to the number of receive antennas, it is necessary to have a number of transmit antennas that is greater than or equal to the number of receive antennas.
The application to digital communications of so-called “time reversal” techniques has made it possible to improve the management of multiple access. Time reversal is a technique for focusing waves (used originally in the field of soundwaves) that relies on the invariance of the wave equation under time reversal. Thus, a time-reversed wave propagates like a forward wave going backwards in time. When a short pulse transmitted from an origin point propagates in a propagation medium, and when a portion of that wave as received by a destination point is time-reversed before being sent back into the propagation medium, the wave converges on the origin point where it reforms as a short pulse. The signal picked up at the origin point is practically identical in its waveform to the original signal transmitted from the origin point.
The time reversal technique is applied to radio communications networks in order to cancel the effect of the propagation channel on the signal received by the receive antenna, in particular by reducing the spreading of the channel by concentrating its energy on a focal point at which the receive antenna is located, and in order to simplify the processing of the symbols that are received after they have been conveyed by the channel. To do this, the antenna signal transmitted by the transmit antenna is pre-equalized by applying coefficients obtained from the time reversal of the impulse response of the propagation channel that is to convey the antenna signal. Performing time reversal thus requires the transmit antenna to have knowledge about the propagation channel in the frequency band dedicated to the signals coming from that antenna.
The application of time reversal techniques to managing multiple access makes use of the fact that the propagation channel between the transmit antenna and a receive antenna possesses a unique electromagnetic signature that can be used for the purposes of separating messages addressed to different receive antennas.
Thus, one approach that makes use of time reversal is proposed in patent application GB 2 463 508 in the context of transmission in blocks, such as OFDMA. That patent application discloses a multiantenna radio communications method in which each respective user antenna communicates with a base station on a frequency subband that has been respectively allocated thereto by means of a signal that has been filtered by time reversal. That multiple-access technique can be considered as being a compromise between frequency division access and pure time reversal access. Its multiband aspect makes it possible to manage signal separation that time reversal on its own cannot manage correctly.
That technique reduces the complexity of receivers compared with multiband methods that do not use time reversal filtering, such as conventional FDMA or the above-described “multiband IR-UWB” technique: in this situation, the signal is by definition pre-equalized and time-focused so that on reception, demodulation is simpler and in particular channel equalization is simpler (as seen by the receiver, the channel is quasi-Gaussian). That technique also reduces the complexity of transmitters relative to known pre-equalization methods that do not make use of time reversal.
Nevertheless, that technique of patent application GB 2 463 508 presents the same drawback as other FDMA type techniques, namely poor spectrum efficiency.